It applies to the detection of X-rays having traversed a device or an object according to a predetermined cutting plane; the detector supplies signals which are, for example, processed by a computer to obtain the image of the cutting of this device or this object. Of course, the detector of the invention can be used in radiography, which is a simplified form of tomography.
The method is known on how to obtain an image of a cutting of a device or an object by means of a tomographic device by transmitting towards this device or object an incident X-ray plane beam, this beam having a wide angular aperture and not having a large thickness. The tomographic detector which is used in this apparatus makes it possible to measure the absorption of X-rays traversing the device or object, this absorption being linked to the density of the tissues of the examined device or to the density of the materials constituting the examined object.
After a suitable digital processing, a multiplicity of scannings in crosswise directions makes it possible to know the signals collected on the detection cells of the detector, the value of absorption of the X-rays at each point of the cutting plane in question, and also the density of the tissues of the device or density of the materials constituting the object. A knowledge of the different values of this density enables the cutting image of the device or object to be restored.
There currently exist an X-ray tomographic detector with a simple structure, but this is not perfect. This detector comprises a sealed chamber containing a gas able to be ionized by incident rays originating from the device or object and, inside the sealed chamber, a polarization plate. This plate is parallel to the beam plane of the incident rays and is brought, for example, to a positive high voltage. A set of load collection electrodes, of the same sign as that of polarization of the plate and resulting from ionization of the gas by the X-rays originating from the object, is disposed opposite said plate. These load collection electrodes (or measuring electrodes), which are flat and elongated, are brought to a potential similar to a reference potential. These electrodes are carried by an insulating substrate and are orientated towards an input gate of the chamber receiving the incident X-rays coming from the object or the device. Each measuring electrode, together with the part opposite the polarization plate, defines an elementary cell of the detector. Each electrode supplies a testing current proportional to the amount of loads obtained by ionization of the gas opposite this electrode under the effect of the rays derived from the object or device. The testing currents are measured by measuring means connected to the electrodes.
In this known type of detector, the polarization plate is fed by an electric voltage source outside the chamber; and the means for measuring currents circulating inside the electrodes are themselves outside the chamber. Thus, connection means are provided which traverse the sealed chamber so as to allow the plate to be fed and the electrode currents to be measured. These connection means may be of the two following types:
connecting wires linking respectively the electrodes and the plate to connection terminals isolated from the chamber and situated on a back face of the latter opposite the gate. These connection terminals or binders are then linked by means of other connecting wires to the power supply and the measuring means;
connecting wires connecting the plate and each of the electrodes respectively to supply and measuring lines embedded in an insulated flexible strip opening inside the chamber between the bottom of the latter and a cover which seals it.
So as to keep a permanent gap between the polarization plate and the substrate which supports the measuring electrodes, means are known for inserting between this plate and the substrate an open-worked insulation brace for passage of the gas having the form of a frame and which slightly absorbs the X-rays.
This known type of detector has many drawbacks. The main drawback results from the fact that the passage of the X-rays inside the insulated brace ionizes the material constituting the brace. Consequently, in the absence of any special precaution, electric charges accumulate inside this material and on its surface and flow through the electrodes towards the measuring means where they appear as strays. These strays are not constant and locally depend on the intensity of the X-rays received and the electrical characteristics of the insulated material constituting the brace. This material, which is usually made of epoxy glass, exhibits a viscosity phenomenon of relatively high duration incompatible with the scrutinizing period of the measurement channels. Moreover, the detection unit partially occupies the chamber and thus there exists dead volumes between the volume occupied by the detection unit and the internal volume of the chamber inside which there is an electric field between the polarization plate and the connecting wires. The configuration of this field is not clearly defined. Consequently, stray electric loads flow into the dead volumes of the chamber and generate eddy current which adversely affect the quality of the tomographic image it is desired to obtain. This stray load detection in fact induces apparent variations of sensitivity of the detection cells; in the images obtained after processing of the testing currents supplied by the cells, these variations are expressed by artifacts which spoil the quality of the image.
Moreover, the electric loads, which may accumulate on the surface of the insulated material constituting the brace, induce a modification of distribution of the potential lines of the electric field in the detection cells; as a result, there may be a risk of disruptive discharge occurring between the measurement electrodes and the polarization plate.
Another drawback results from the accumulation of current loads, especially on the surface or inside the brace insulating material; this accumulation modifies perpendicularity of the electric field with respect to the electrodes and uncontrollably develops the collection rate of loads created in the gas.
Finally, a further drawback of this known type of detector results from connecting the plate to electric power supply external means and from connecting the electrodes to external measurement means. These connections, which require the use of connecting wires connected to the binders or to an antenna of conductors opening onto the back surface of the chamber, is difficult to embody. Moreover, these conducting wires are distributed inside the chamber, this distribution being difficult to control.